1. Field of the Invention
The present invention relates to a system and method for detecting two or more targets positioned at different sites and, in its preferred use, to an optical device which tests laser rangefinders in which spaced apart simulated optical range targets produce multiple reflections from a single input pulse of electromagnetic energy.
2. Description of the Prior Art
While the present invention has particular application to testing of optical rangefinders, it is to be understood that it is equally useful to detect a plurality of targets positioned at different sites. Nevertheless, specific discussion will be directed toward such testing, for which the invention was specifically devised.
Inventions relating to such rangefinder testing are described in U.S. Pat. Nos. 4,068,952 patented Jan. 17, 1978 and 4,121,890 patented Oct. 24, 1978. As stated in the former patent, rangefinders, specifically laser rangefinders, have been conventionally tested in outdoor target ranges. Such testing is generally inconvenient and/or hazardous. The equipment must be moved to the range where personnel may be harmed as a result of accidental exposure to the laser beam. In addition, the environment produces variations of optical transmissivity from moisture, dust and other pollutants. In was thus an object of the invention of U.S. Pat. No. 4,068,952 to eliminate such inconvenience and hazards by bringing a laser rangefinder test unit to the laser itself.
The optical range tester described therein includes a coil of an optical waveguide, such as a single optical fiber, input and output focusing lenses and a multiple tapping unit for extracting or sampling light energy from the fiber at a number of predetermined points along the coil to provide different time delays corresponding to minimum, nominal and range resolution returns. All of the sampled output beams are combined optically so that they leave the device on a common optical axis so as to be sensed by an optical detector in the laser receiver. The multiple tapping unit includes a reflection system coupled to a discontinuity in the optical fiber which forms an input and an output. Diffused energy from the input fiber is reflected for sampling, while the remainder of the energy continues to down line sampling stations through the output fiber. That system has worked well in providing a capability to detect such closely placed targets.
An alternate sampling method for reflecting a portion of the laser energy to provide at least dual target capability is provided by splicing together two spools of fibers having cores of different diameters. The spool closest to the furthest target has the smaller diameter core so that a reflection is provided by the core-to-cladding interface at the splice junction. Such a splice is difficult to produce and produces stress in the fiber, leading to breakage.
The greatest disadvantage to such a splice is that it is very inefficient. The reflection coefficient is 0.05%, with 74% of the energy being absorbed, and only 25% of the energy being transmitted. The transmitted light reflected from the furthest placed target to the rangefinder has a reflection coefficient of 4%.
Such inefficiency in transmission of the energy produces a situation in which the thick lens collimator described in U.S. Pat. No. 4,121,890 can be and has been damaged. Energy is transmitted at a relatively high level and, to prevent it from damaging the collimator, it is conventional to decrease the energy by placing a dense filter in front of the collimator. However, because of the energy loss in the splice, the intensity of the laser energy cannot be decreased to too great an extent; otherwise there would be too little energy transmitted, and reflected back, through the splice for proper test system response. Accordingly, a less dense filter is employed which exposes the collimator to potential damage.